The MRI apparatus is an apparatus that measures an NMR signal (echo signal) generated by an object, especially, the nuclear spins that form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding and different frequency encoding are given to NMR signals according to the gradient magnetic field, and the NMR signals are measured as time series data. The measured echo signals are reconstructed as an image by two-dimensional or three-dimensional Fourier transform.
Echo signals are measured according to the imaging sequence set in advance. There are a variety of imaging sequences. Among them, there is an imaging sequence that increases the speed of collecting echo signals by applying a refocus high frequency magnetic field (RF) pulse multiple times while changing the amount of phase encoding after applying an excitation high frequency magnetic field (RF) pulse once. Such an imaging is called multi-echo imaging, and a group of echo signals measured after each refocus RE pulse is called an echo train. In addition, the number of echo signals obtained after one RF pulse application is called an echo train length (ETL).
In multi-echo imaging, the flip angle (FA) of the refocus RF pulse may be changed. Such an imaging sequence is called a VRFA multi-echo sequence. In the VRFA multi-echo sequence, the FA of each refocus RF pulse is determined so as to achieve a predetermined purpose.
For example, there is a technique for determining the FA so that the resolution of an image is increased (for example, refer to PTL 1). In the technique disclosed in PTL 1, the FA is determined so that the signal strength of each echo signal in the echo train is constant. This is because signal attenuation is suppressed if the signal strength of each echo signal is constant and accordingly, there is no image blur (resolution is increased) even if the ETL is increased. Typically, the FA in PTL 1 steadily increases toward the final FA after becoming a small value (FAmin) once. Here, the FA at the center of k-space is defined as FAcenter, and the final FA is defined as FAmax.
In addition, in this specification, it is assumed that strength changes due to phase encoding and frequency encoding be neglected in the signal strength of the echo signal. In addition, the value of the signal strength is normalized with the virtual signal strength defined as 1 at the moment of excitation.
However, the image quality is determined not only by the resolution but also by various factors, such as the insensitivity to movement and the signal to noise ratio (SNR). In particular, the SNR is important in determining the image quality. Since the intention of the technique disclosed in PTL 1 is to make the signal strength constant, the SNR of the obtained image does not necessarily become the best. In addition, the method of determining the value to be made constant is arbitrary, and the image quality to be obtained at a certain value is not known.
In order to improve the SNR, there is a technique for determining the FA so that the signal strength of echo signals arranged at the center of k-space is increased (for example, refer to NPL 1). This is because echo signals arranged at the center of k-space generally determine the SNR. In NPL 1, FAmin, FAcenter, and FAmax are designated and then the FA is determined so that the signal changes steadily. This is because the insensitivity to movement in the image can be increased by increasing the FAmin and the SNR when signal correction is not taken into consideration can be improved by increasing the FAcenter.
In addition, there is a technique for determining the FA so that the signal strength of each data item is aligned when calculating images of two types of contrast by adding and subtracting (combining) two types of data, (for example, refer to NPL 2). In NPL 2, data whose balance of the signal strength of the brain parenchyma and CSF (cerebrospinal fluid) is different is acquired as the two types of data. Images of T2W and FLAIR are obtained by adding and subtracting these data items. In this case, the FA is determined so that the signal strength of the CSF of the two types of data is aligned in a region of the center of k-space. By aligning the signal strength when two types of data are acquired, weighting for aligning the signal strength at the time of addition and subtraction is not required. As a result, the SNR is improved.
In addition, an extended phase graph (EPG) is known as a method of calculating a change in the signal strength from the determined FA (for example, refer to NPL 3). In addition, conversely, a Prospective EPG is known as a method of calculating the FA from the change in the signal strength (for example, refer to NPL 4).